[gdb/cli] Fix help text of maint set ignore-prologue-end-flag

I noticed here:
...
(gdb) help maint set ignore-prologue-end-flag
Set if the PROLOGUE-END flag is ignored.
The PROLOGUE-END flag from the line-table entries is used to place \
  breakpoints past the prologue of functions.  Disabeling its use use forces \
  the use of prologue scanners.
...
a typo in "Disabeling" and accidental word repetition "use use".

Fix by replacing with "Disabling" and "use".

Reviewed-By: Tom Tromey <tom@tromey.com>